We are attempting to isolate and characterize an antigen(s) from human synovial cells which may be unique for synovial cells of rheumatoid origin. The major emphasis at this time is the delineation of rabbit anti-synovial cell antibody as to its specificity. Characterization of such an antibody has been recently described. (Jameson A, Steffes ML, Martincic R et al. Characteristics of an antigen reactivity in synovial fluids and its association with rheumatoid arthritis. PSEBM 155:318-323, 1977). Our preliminary data confirm those reported. We have now raised antibodies to both rheumatoid and nonrheumatoid synovial cells and whereas early results suggested different specificities, it now appears the antibodies may have a very similar spectrum of antigenic recognition.